1. Field of the Invention
The present invention relates to a piezoelectric material, in particular, to a lead-free piezoelectric material. The present invention also relates to a piezoelectric element, a multilayered piezoelectric element, a liquid discharge head, a liquid discharge apparatus, an ultrasonic motor, an optical apparatus, a vibratory apparatus, a dust removing apparatus, an image pickup apparatus, and an electronic apparatus that each include such a piezoelectric material.
2. Description of the Related Art
Lead zirconate titanate, which contains lead, is a typical piezoelectric material and is used in various piezoelectric devices such as actuators, oscillators, sensors, and filters. After such piezoelectric devices are discarded, the lead component of the piezoelectric material may migrate into the soil, causing harmful effects on the ecosystem. For this reason, in order to provide lead-free piezoelectric devices, lead-free piezoelectric materials are being intensively studied.
It is required that the piezoelectric performance of piezoelectric elements used in products such as household appliances does not considerably vary within the operation temperature ranges of the products. When parameters relating to piezoelectric performance, such as an electromechanical coupling factor, a dielectric constant, a Young's modulus, a piezoelectric constant, a mechanical quality factor, and a resonance frequency, considerably vary (for example, by 30% or more) in response to changes in temperature, stable element performance within the operation temperature ranges is difficult to provide. During phase transition of piezoelectric materials, the piezoelectricity becomes maximum at the phase transition temperatures. Thus, phase transition is the largest factor that causes variations in the piezoelectric performance. Accordingly, when stable piezoelectric performance within the operation temperature ranges of products is intended, piezoelectric materials that do not have phase transition temperatures within the operation temperature ranges are required.
Piezoelectric compositions used in resonant devices such as ultrasonic motors desirably have a high mechanical quality factor, which represents the degree of resonance sharpness. When the mechanical quality factor is low, a high power is required for operation or the piezoelectric element generates heat and driving thereof becomes difficult to control. For these reasons, there have been demands for piezoelectric materials having a high mechanical quality factor.
Japanese Patent Laid-Open No. 2009-215111 (hereafter Patent document 1) discloses a material represented as a pseudo-binary solid solution {[(Ba1-x1M1x1)((Ti1-xZrx)1-y1N1y1)O3]−δ% [(Ba1-yCay)1-x2M2x2)(Ti1-y2N2y2)O3]}(M1, N1, M2, and N2 are added elements). (Ba1-xM1x1)((Ti1-xZrx)1-y1N1y1)O3 forms a rhombohedral crystal. (Ba1-yCay)1-x2M2x2) (Ti1-y2N2y2)O3 forms a tetragonal crystal. By dissolving, in each other, two components having different crystal systems, the temperature of phase transition between the rhombohedral crystal and the tetragonal crystal is adjusted to be about room temperature. Patent document 1 discloses that, for example, BaTi0.8Zr0.2O3−50% Ba0.7Ca0.3TiO3 has a phase transition temperature that is about room temperature and has at 20° C. a piezoelectric constant d33 of 584 pC/N and at 70° C. 368 pC/N. That is, compared with the piezoelectric constant d33 at 20° C., a 40° C. increase in the temperature results in a 37% decrease in the piezoelectric constant d33. The piezoelectric material of Patent document 1 undergoes, at room temperature, phase transition at which piezoelectricity becomes maximum and hence has high piezoelectric performance at about room temperature; however, the piezoelectric performance of the piezoelectric material considerably varies in response to changes in temperature. In the piezoelectric material, the Zr proportion (x) in (Ba1-x1M1x1)((Ti1-xZrx)1-y1N1y1)O3 serving as an end member is set to be more than 0.1 to form a rhombohedral crystal.
Karaki, 15th US-Japan Seminar on Dielectric and Piezoelectric Ceramics Extended Abstract, p. 40 to 41 discloses a piezoelectric ceramic obtained by firing, by two-step sintering process, BaTiO3 mixed with 0.03 parts by weight of Mn and 0 to 0.3 parts by weight of LiBiO2. As a result of addition of LiBiO2, the coercive electric field of BaTiO3 mixed with 0.03 parts by weight of Mn substantially linearly increases with the amount of LiBiO2 added and the piezoelectric constant d33, dielectric constant, and dielectric loss tangent of BaTiO3 similarly decrease. When 0.17 parts by weight of LiBiO2 is added, the piezoelectric constant d33 is 243 pC/N and the coercive electric field is 0.3 kV/mm. When 0.3 parts by weight of LiBiO2 is added, the coercive electric field is 0.5 kV/mm. However, the inventor of the present invention performed thorough studies and, as a result, has found that this piezoelectric material undergoes phase transition between a rhombohedral crystal and a tetragonal crystal within the temperature range of 5° C. to −30° C.; and the piezoelectric material has a low mechanical quality factor of less than 500 at room temperature.
The existing techniques are disadvantageous in that the piezoelectric performance of piezoelectric ceramics considerably varies within the operation temperature ranges of piezoelectric elements, and the piezoelectric ceramics have a low mechanical quality factor.
The present invention provides a piezoelectric material that does not contain lead, that does not undergo phase transition within the operation temperature range, and that has a high density, a high mechanical quality factor, and good piezoelectric properties. The present invention also provides a piezoelectric element, a multilayered piezoelectric element, a liquid discharge head, a liquid discharge apparatus, an ultrasonic motor, an optical apparatus, a vibratory apparatus, a dust removing apparatus, an image pickup apparatus, and an electronic apparatus that each include the piezoelectric material.